The present disclosure generally relates to a dual modality detection system for detecting gamma rays and neutrons associated with the presence of nuclear material in containers.
There is a growing concern that terrorists or others may attempt to import into the United States or some other country radioactive or nuclear material, which may then be used for the construction of a nuclear weapon for carrying out terrorist objectives. One way of terrorists can ship such radioactive or nuclear material is to hide the material among or within seemingly innocuous cargo containers. For example, such nuclear material could be placed within a standard, sealed cargo container of the type typically employed for shipping cargo by sea, rail, air, or truck. The nuclear material could be positioned in such a sealed cargo container along with other innocuous goods with the container being positioned, for example, within the hold of a large container ship which may be transporting a thousand or more such containers from one location to another. The entire contents of a seagoing cargo container can have a mass as large as 27 metric tons, can span the range of all materials that are common to international commerce, and can range from homogeneous to heterogeneous loads with random or regular voids. As such, detection of radioactive or nuclear material contained in such containers can be a daunting and difficult task.
One of the problems inherent to detecting the relatively small amounts of radioactive or nuclear material hidden within larger masses of other material in containers is the attenuation of the radiation signature and/or the attenuation of the interrogating radiation by intervening cargo. Other problems are the presence of background radiation from which the signal must be distinguished and the limited amount of inspection time available for the development of a significant signal.
Because of the thickness of the intervening walls in containers and the cargo contained therein, which can be greater than 1 meter or more, highly energetic radiation such as gamma rays and neutrons can penetrate this thickness. As such, nuclear inspection methods are generally divided into two classes: passive and active. Passive inspection methods are employed to detect nuclear materials that are naturally radioactive such that their intrinsic radiations may provide a useful signature. In contrast, active inspection methods, also referred to as active interrogation, generally include injecting high-energy neutrons or photons (x-rays or gamma rays) into the container to cause a the nuclear material contained therein to undergo fission and emit characteristic high-energy neutrons and/or gamma rays that can then be detected outside the container. In this manner, the active interrogation method is nondestructive and uses penetrating nuclear radiation, such as neutrons or photons, as a probe to stimulate a unique radiation signature that will identify a material or characteristic of interest.
Current equipment to determine the presence of radioactive or nuclear material employ a stand-alone “gamma ray” detection device for detection of gamma rays (e.g., detecting the number of counts, or gammas) or a stand-alone neutron detector for detecting neutrons (e.g., delayed neutrons). Gamma rays excite a gamma ray detector by interacting with its electrons to cause ionization.
Neutrons, however, are neutral particles that don't interact appreciably with electrons but do undergo reactions with the nuclei of atoms. Neutron detection then becomes a two-step process. The sensitive material of a neutron detector contains atoms with nuclei that are highly susceptible to reacting with neutrons. The nuclear reactions produce energetic charged particles that interact with electrons and produce the necessary ionization needed for detection.
Fast neutrons can be detected employing high-energy interactions with charge particles, which generate the detection signal. Fast neutrons can also be detected by slowing them down to thermal energies with a moderator and using a material with a high capture cross section to produce the detected signal. The sensitive material could be a gas such as 3He or 10BF3 encapsulated in tubes, or it could be a substance such as gadolinium embedded in a suitable medium, e.g., a scintillating liquid or scintillating fiber.
FIGS. 1 and 2 illustrate a prior art detection system 10 that includes an exemplary stand-alone gamma ray detector 12 and an exemplary stand-alone neutron detector 14. The gamma ray detector 12 generally includes a scintillator 16 and a photodetector 18 configured for detecting gamma rays. The scintillator 16 is generally formed of a substance that interacts with high energy (ionizing) electromagnetic or charged particle radiation then, in response, fluoresces photons at a characteristic Stokes shifted (longer) wavelength, releasing the previously absorbed energy. The photodetector 18 is used to detect the light emitted by the scintillator 16.
The illustrated neutron detector 14 generally includes a moderator 22 and tubes 24 filled with a He-3 gaseous medium 26 for detecting neutrons. The moderator 22 is generally used to reduce the energy of the neutron so that they can interact with the He-3 gaseous medium in a manner well known in the art. The moderator can be formed of a plastic such as polyethylene.
Because the detectors are generally configured to detect relatively weak emission signals of nuclear material deeply hidden within containers, relatively large detectors are required. Moreover, since the current state of art uses stand-alone detectors, i.e., one for gamma ray detection and one for neutron detection, the process and equipment can become expensive, complex and require a large footprint.
Accordingly, there is a need in the art for a dual modality detection system that provides a smaller footprint and can provide simultaneous detection of the gamma ray and neutron signatures of nuclear material hidden within containers with minimal false alarms.